The present invention relates to a receiver for a broadcasting service and particularly to a digital broadcasting signal receiver.
As a digital broadcasting service has recently been initiated with a ground wave, a digital modulation technique is required. Such a digital technique of modulating ground wave broadcasting signals commonly consists of superimposing multi-level data over a signal amplitude for increasing the transmission efficiency per unit frequency. In a digital broadcasting receiver, a received digital modified signal is digitally demodulated to a binary signal. It is, however, necessary for improving the quality of the binary signal defined by a bit error rate (BER) to precisely control the amplitude of the digital demodulated signal to a desired level, i.e., carry out automatic gain control.
FIG. 15 illustrates a conventional digital broadcasting signal receiver employing an automatic gain controller. As shown, an RF signal received at an antenna (not shown) is introduced into a mixer 1 where it is combined with an output of a local oscillator 2 to yield an IF signal. A desired frequency band is extracted from the IF signal by a Band Pass Filter (BPF) 3 and transferred to a gain-variable amplifier 4.
The IF signal at mixer 1 is tuned by the frequency of the output of local oscillator 2 so that the IF signal can contain a particular channel signal which is selectively picked up.
The IF signal shaped by variable-gain amplifier 4 is further transferred to a digital signal demodulator 5 where the IF signal is digitally demodulated to generate a binary signal.
Variable-gain amplifier 4 and digital signal demodulator 5 form a feedback control loop together with an automatic gain controller 6 which consists mainly of an automatic gain control device 7, a loop filter 8, and a loop amplifier 9.
Automatic gain control device 7 detects a difference in the amplitude between the IF signal and the desired channel signal from the output of digital signal demodulator 5. The amplitude difference is smoothed by loop filter 8, amplified by loop amplifier 9, and fed back as a control signal to variable-gain amplifier 4 where it controls the signal amplifying rate, thus, to compensate for the amplitude difference in the IF signal.
Meanwhile, it is essential for any digital broadcasting signal receiver to establish the compatibility between quick tuning of a desired channel signal and use of higher BER. The reception of ground wave signals from ground broadcasting stations, unlike the direct transmission of satellite broadcasting signals from satellites, depends largely on the location of an antenna of the digital broadcasting receiver. As the transmission of ground wave signals is commonly disturbed by external obstacles, a received signal may contain static ghosts or Doppler ghosts, hence, declining BER.
In particular, any object located between the transmitter of a broadcasting signal and the antenna of the receiver (for example, the leaves or the branches of trees swayed by winds) may generate an external disturbance known as target wave fluctuation, where the amplitude (intensity of electric field) of a received signal is varied from time to time, hence, significantly declining BER and impairing the reproduction of images. The target wave fluctuation may cause a change of several decibels in the amplitude at intervals of several tens of hertz.
Since the band-width of the loop filter and the gain of the loop amplifier in the automatic gain controller of the conventional digital broadcasting signal receiver are fixed, it is required to increase the band-width of the loop filter and to broaden the control frequency range of the automatic gain controller for tuning a desired channel at a short period of time, thus enhancing the frequency response of the automatic gain controller. Alternatively, the control frequency range can be broadened by increasing the gain in the feedback control circuit. As the gain of the loop amplifier is increased, the control frequency range of the automatic gain controller expands to enhance the frequency response of the automatic gain controller. By enhancing the frequency response of the automatic gain controller, the amplitude of the IF signal introduced to the digital signal demodulator is quickly settled down to a desired range of a level, hence permitting the tuning to a desired broadcasting signal at a shorter period of time.
However, as the control system generally tends to be responsive to a too high frequency range including Doppler ghosts, the amplitude of an input signal to the digital signal demodulator may not stay in the desired range, thus declining the BER characteristics and deteriorating the performance of the digital broadcasting signal receiver.
On the contrary, when the band-width of the loop filer is narrowed to limit the control frequency of the automatic gain controller or the gain of the loop amplifier decreases to limit the control frequency range of the automatic gain controller, a control frequency range of the automatic gain controller becomes smaller than the frequency range of amplitude variation of the IF signal. As a result, the gain controller can hardly respond to any target wave fluctuation particularly with the antenna of the digital broadcasting signal receiver located in a region where the target wave fluctuation occurs frequently. This causes the amplitude of an input signal introduced to the digital signal demodulator not to stay in a desired level, hence declining the BER characteristics, seriously disturbing the production of images, and degrading the performance of the digital broadcasting signal receiver.
A digital broadcasting signal receiver has a compatibility between the tuning to a desired channel in a short period of time and an improvement of the BER characteristics, and in a case where a target wave fluctuates, prevent significant declination of the BER characteristics and ensure the reproduction of quality images.
The digital broadcasting signal receiver comprises a synchronous state discriminator or an electric field intensity detector for detecting a change in the intensity of an electric field of a received IF signal, so that the frequency range of a loop filter or the gain level of a loop amplifier in an automatic gain controller can be switched according to an output of the synchronous state discriminator or the electric field intensity detector.
The receiver consequently permits the frequency band of the loop filter or the gain level of the loop amplifier in the automatic gain controller to be selectively adjusted to one setting designed for minimizing the duration for tuning a desired channel before the synchronous state is established. Then, after the synchronous state is established, the frequency band of the loop filter or the gain level of the loop amplifier in the automatic gain controller is switched to another setting designed for enhancing the BER characteristics. As a result, the minimum duration for tuning to the desired channel and the higher BER rate will be compatible with each other.
Also, the digital broadcasting signal receiver allows the electric field intensity detector to monitor the amplitude of the intensity of the electric field of the received IF signal. When an amplitude change in the intensity of the electric field and the varying frequency exceed predetermined levels, it is judged that a target wave fluctuation occurs, and the frequency band of the loop filter or the gain level of the loop amplifier in the automatic gain controller is selectively switched. This limits the amplitude of the signal introduced to the digital signal demodulator to a desired range regardless of the occurrence of a target wave fluctuation in the antenna region of the digital broadcasting signal receiver, hence preventing significant declination of the BER characteristics and ensuring the reproduction of high quality images.